DNA Repair Defects in IgA Deficiency Immunoglobulin A deficiency (IgAD) is the most common primary immunodeficiency in humans, and it is characterized by the absence of serum IgA and an increased frequency of bacterial infections in the respiratory and gastrointestinal tracts. IgAD may develop over time into a more severe form of antibody deficiency, common variable immunodeficiency (CVID), and the two diseases can even co-exist in different members of the same family suggesting that they represent facets of the same underlying genetic defect(s). In order for IgA to be produced, the expressed heavy chain antibody isotype must be swapped for DNA sequences that encode IgA. This DNA level transaction is called class switch recombination (CSR) and it occurs between two so-called switch region sequences that reside upstream of the respective exons that encode the constant region of the antibody heavy chain. Recent studies have demonstrated that CSR is initiated by AID-catalyzed DNA cytosine deamination events and that the resulting uracil residues are recognized and processed by ubiquitous DNA repair systems such as base excision repair, mismatch repair, non-homologous end-joining and, most recently, single-strand annealing (alternative end-joining pathway). Defects in AID or uracil excision (UNG2) cause an immunodeficiency characterized by high levels of IgM at the expense of other antibody isotypes including IgA. We recently discovered variations in the mismatch repair gene MSH5 that associate with IgAD and CVID. Subsequent preliminary studies in our laboratories have revealed variations in other DNA repair genes that also associate with CVID and/or IgAD. This proposal will build on our prior studies by directly testing the hypothesis that multiple DNA repair defects also contribute to IgAD/CVID. The complete repertoire of known DNA repair genes from a minimum of 400 IgAD/CVID patients will be sequenced by a new technology called massively parallel DNA sequencing or 454 pyrosequencing. After computational analyses, candidate mutations will be validated by re-sequencing and by family studies using the largest IgAD/CVID patient cohort in the world. Key disease-associated mutations will be examined in cell-based systems for defects in DNA repair and in antibody gene diversification program. Overall, we anticipate significantly building upon our prior studies by demonstrating the involvement of several additional DNA repair factors in IgAD/CVID. PUBLIC HEALTH RELEVANCE: Antibodies are a crucial part of our immune response to pathogens such as bacteria and viruses. Recent evidence from many laboratories indicates that ubiquitous DNA repair factors help ensure an effective antibody response by facilitating the DNA level transactions that are required to generate a diverse and strong antibody repertoire. This proposal will determine the complete set of DNA repair gene sequences for more than 400 patients from one of the largest immunodeficiency disease cohorts in the world. We anticipate discovering several new factors that are essential for a robust antibody responses to pathogens.